Trait-based assembly rules across climatic gradients of European grasslands are affected little by extreme drought

Changes in climate such as extreme drought events might interfere with grassland ecosystem processes. European grasslands support a rich flora with high small-scale species density, mirroring complex mechanisms of coexistence in different habitat templates. Do these mechanisms differ in relation to climate, soil and land use history? Are they affected by extreme drought? We investigated fine-scale patterns of trait-based community assembly in European grasslands across continental and experimental climatic gradients within the framework of the BiodivERsA project SIGNAL. The gradient extends from mesic (FR, GE) to intermediate (IT, BG) and to xeric grasslands (TR, HU). The sites also differ in management, disturbance history, geology and edaphic factors and represent common grassland-types of the respective country. We sampled fine-scale patterns of species combinations (rooting individuals) in six 1.20 m X 0.40 m blocks, subdivided into micro-quadrats of 100 cm2, for each treatment (control, drought) at each site in two years (2013, 2014). Based on the key plant-traits specific leaf area (SLA), height and seed mass we calculated Rao’s functional diversity (FD) and further diversity indices for each micro-quadrat and compared them to a null model using the randomization method of Schamp et al. (2008; Journal of Ecology 96: 204–212); deviation of FD from random expectation was interpreted as trait divergence or convergence. Strong seed-mass divergence in the dry Turkish grassland pointed towards complex disturbance history, while the other countries showed convergence. Plant-height convergence most likely indicated the importance to have similar best traits in productive grasslands (FR, GE) or trait-similarity due to environmental filters (HU, IT) as does the strong SLA-convergence in Italy. We conclude that assembly rules can be contrasting and context dependent at different grassland sites and climatic differences are often masked by local factors such as disturbance regime or soil heterogeneity. Experimental extreme drought led to a tendency towards trait-convergence in seed mass in Italy probably constraining the regeneration niche. The tendency of SLA-divergence in France could indicate a tempering of competitive situations. Apart from that, assembly rules were remarkably stable under drought as we found neither significant changes in trait divergence or convergence nor switches between them across countries.